Increasingly, transcutaneous electrical nerve stimulation (TENS), also often referred to as electronic pain control, is finding acceptance as an aid or supplement to anesthesia and/or analgesia. It has been known for some time that transcutaneous stimulation is a useful and effective moderator of post-operative pain (see, for example, U.S. Pat. No. 3,911,930 to Hagfors et al.). More recent investigations indicate that TENS may also have a synergistic interaction with oft used anesthetics and analgesics in a fashion which substantially alleviates pain and discomfort of the patient, while substantially reducing the amount and character of drugs to which the patient must be subjected. In U.S. application Ser. No. 133,211 to Bussey, filed Mar. 24, 1980, there is disclosed a method for utilizing the combination of transcutaneous nerve stimulation with anesthetics and analgesics during surgery. That application also features the utilization of electronic pain control apparatus in such a manner and of such a character as the electrodes which are positioned on the patient before and during surgery, may be left in place for utilization for alleviating pain during the post-operative period.
Quite commonly, however, both during the operative procedure and during the recovery period thereafter, the surgeon, the anesthesiologist, and other attending physicians and nurses have need continuously to monitor certain vital signs of the patient, such as cardiac function. The conventional mode of monitoring cardiac function is utilization of the electrocardiograph (EKG) through the positioning of several sensing electrodes on the patient, in known fashion, thereby to detect induced electrical variations proportional to and directly representative of heart activity (i.e. the well-known cardiac PQRST complex).
Problems arise in the simultaneous conduct of transcutaneous electrical nerve stimulation and the likes of EKG monitoring, however, because of the character and frequency spectrum of the EKG signal, in both healthy and ailing patients, and the character and frequency spectrum of electrical signals which are known to be effective for purposes of electronic pain control. Unless some appropriate precaution is taken, the EKG trace during and immediately after each pulsed application of TENS at best will be meaningless, and at worst will provide a false or erroneous indication of patient conditions which in fact are not occurring.
It is accordingly a primary object of the present invention to provide apparatus and methods whereby transcutaneous electrical nerve stimulation may be applied to a patient, either alone or in combination with select anesthetic and analgesic agents, during times in which patient vital signs, such as cardiac function, are being continuously or intermittently monitored.
Problems attendant to mutually interfering signals both generated in treating the patient and sensed in the patient through monitoring, are not new. Indeed, a number of patents purport to deal with such mutual interference problems in the operating theatre, in the intensive care unit, and elsewhere. For example, U.S. Pat. No. 4,117,848 to Naylor discloses the use of a follow and hold circuit to suppress pacer pulses in an EKG monitor. Likewise, U.S. Pat. No. 3,897,774 to Burdick et al., U.S. Pat. No. 4,137,908 to Degonde et al., and U.S. Pat. No. 3,534,282 to Day disclose alternative approaches to mutually noninterfering utilization of heart pacing and EKG monitoring. U.S. Pat. No. 3,716,059 to Welborn et al. describes a cardiac resuscitator including a disposable clamp controlled to inhibit transmission of EKG signals at times when electrical stimulation is being delivered to the patient. Fletcher U.S. Pat. No. 3,910,257 discloses a sample and hold approach to EKG signals, for purposes of monitoring and subsequent coupling to a data acquisition unit.
Each such prior art approach, is less than preferred for the electronic pain control situation, not to mention for their own purported situation, for a variety of reasons. Not the least of these is the need externally to interconnect the respective signal generating and signal sensing units; also they tend to sense signals in a fashion which is counterproductive to overriding TENS objectives, or which involves specialized and elaborate synchronization and signal processing requirements, or which is so especially adapted to the application disclosed in the patents that there results little of relevance to the TENS-EKG situation.